1. Field of the Invention
The present invention relates to a displacement detector for detecting the amount of displacement in rotation, linear motion, or the like of an object to be measured, in particular, to a displacement detector which is capable of accurately detecting the amount of displacement in absolute value even under adverse circumstances.
2. Description of the Prior Art
Heretofore, in the rotary encoder which is a kind of displacement detector, the absolute value of the rotation angle of rotation shaft is obtained as a digital signal in the following manner. Namely, as shown in FIG. 1, a disk-shaped coded plate 10 having digital patterns that consist of slits or apertures, as indicated by hatches in the figure, at different radial positions corresponding to every predetermined central angles, is mounted coaxially on a rotation shaft 16 that is held with a bearing 14 by a case 12 indicated by the single-chained line in the figure. Then, the absolute value of the coded angle of rotation of the rotation shaft 16 is obtained as a digital signal in a detection unit 18, by irradiating the digital patterns with a light source from one side of the coded plates 10 and by detecting the transmitted light with an optical sensor. In order to improve the resolving power of the rotation angle that is detected by such a rotary encoder, it is necessary to increase the number of digits of the digital patterns so that it leads to a problem that the diameter of the coded plate 10 has to be increased and the size of the device has to be made large, resulting in a higher cost for the product. Further, when the number of digits are increased by arranging the digital patterns closely in the radial direction, there is required a higher finishing precision for the light source or the optical sensor in the detection unit 18.
Moreover, the digital patterns in the existing rotary encoder as in the above are composed of Gray code. However, no special measures are being taken regarding possible failure of the device. Now, in an encoder in which a plurality of sensors are arranged in a line with predetermined spacings, for outputting a detection signal that corresponds to the position of the object to be measured that moves along the line, and for converting the detection signal that is output from the sensor to a corresponding individual code, use is made of numerous circuit elements, so that there exists a problem that the probability of failure of the device is accordingly high. This necessitates then the provision of a failure diagnosing means for checking whether or not the device is operating normally.